Seeker guided ordnances are weapons that can be launched or dropped some distance away from a target, then guided to the target, thus saving the delivery vehicle from having to travel into enemy defenses. Seekers make measurements for target detection and tracking by sensing various forms of energy (e.g., sound, radio frequency, infrared, or visible energy that targets emit or reflect). Seeker systems that detect and process one type of energy are known generally as single-mode seekers, and seeker systems that detect and process multiples types of energy (e.g., radar combined with thermal) are generally known as multi-mode seekers.
Seeker homing techniques can be classified in three general groups: active, semi-active, and passive. In active seekers, a target is illuminated and tracked by equipment on board the ordinance itself. A semi-active seeker is one that selects and chases a target by following energy from an external source, separate from the ordinance, reflecting from the target. This illuminating source can be ground-based, ship-borne, or airborne. Semi-active and active seekers require the target to be continuously illuminated until target impact. Passive seekers use external, uncontrolled energy sources (e.g., solar light, or target emitted heat or noise). Passive seekers have the advantage of not giving the target warning that it is being pursued, but they are more difficult to construct with reliable performance. Because the semi-active seekers involve a separate external source, this source can also be used to “designate” the correct target. The ordinance is said to then “acquire” and “track” the designated target. Hence both active and passive seekers require some other means to acquire the correct target.
In semi-active laser (SAL) seeker guidance systems, an operator points a laser designator at the target, and the laser radiation bounces off the target and is scattered in multiple directions (this is known as “painting the target” or “laser painting”). The ordinance is launched or dropped somewhere near the target. When the ordinance is close enough for some of the reflected laser energy from the target to reach the ordinance's field of view (FOV), a seeker system of the ordinance detects the laser energy, determines that the detected laser energy has a predetermined pulse repetition frequency (PRF) from a designator assigned to control the particular seeker system, determines the direction from which the energy is being reflected, and uses the directional information (and other data) to adjust the ordinance trajectory toward the source of the reflected energy. While the ordinance is in the area of the target, and the laser is kept aimed at the target, the ordinance should be guided accurately to the target.
Multi-mode/multi-homing seekers generally have the potential to increase the precision and accuracy of the seeker system but often at the expense of increased cost and complexity (more parts and processing resources), reduced reliability (more parts means more chances for failure or malfunction), and longer target acquisition times (complex processing can take longer to execute). For example, combining the functionality of a laser-based seeker with an image-based seeker could be done by simple, physical integration of the two technologies; however, this would incur the cost of both a focal plane array (FPA) and a single cell photo diode with its associated diode electronics to shutter the FPA. Also, implementing passive image-based seekers can be expensive and difficult because they rely on complicated and resource intensive automatic target tracking algorithms to distinguish an image of the target from background clutter under ambient lighting. Another factor limiting multi-mode seeker performance is the general incompatibility between the output update rate of a semi-active laser-based seeker system and the output update rate of a passive image-based seeker system. In general, the output update rate from an active laser-based seeker to its guidance subsystem is limited to the PRF of the laser designator (typically from 10 to 20 Hz.), whereas the output update rate of a passive, image-based seeker is limited by the frame rate of its imager and available ambient light (typically greater than 60 Hz.).
Because seeker systems tend to be high-performance, single-use items, there is continued demand to reduce the complexity and cost of seeker systems, particularly multi-mode/multi-homing seeker systems, while maintaining or improving the seeker's overall performance.